Materials having such heat resistance as to endure temperatures of 150° C. or above have been demanded as materials for covering semiconductor elements in semiconductor devices which require high heat resistance and high voltage endurance. Among them, materials for covering optical materials such as light emitting diode (LED) elements have been demanded to have, in addition to heat resistance, properties such as flexibility, transparency, resistance to thermal yellowing, and resistance to photoyellowing.
A synthetic polymer compound has been reported as a material having high heat resistance and good heat dissipation (Patent Literature (PTL) 1). The synthetic polymer compound includes at least one third organosilicon polymer having a molecular weight of 20000 to 800000, in which the third organosilicon polymer includes at least one kind of first organosilicon polymer and at least one kind of second organosilicon polymer bonded with each other through siloxane bonds, the first organosilicon polymer has a crosslinked structure connected through a siloxane (Si—O—Si bonded structure), and the second organosilicon polymer has a linear connection structure connected through a siloxane. Such material, however, has properties not yet satisfactory.
A resin composition for sealing an optical element has been disclosed as a resin composition for sealing optical element, which excels in transparency, ultraviolet resistance, and resistance to thermal coloration (PTL 2). This resin composition includes, as a resin component, at least one silsesquioxane selected from the group consisting of a liquid silsesquioxane having a cage structure and containing an aliphatic carbon-carbon unsaturated bond but no hydrogen-silicon (H—Si) bond; and a liquid silsesquioxane having a cage structure and containing a H—Si bond but no aliphatic carbon-carbon unsaturated bond. Cured articles derived from such cage silsesquioxanes, however, are relatively rigid, have poor flexibility, and are susceptible to cracking or fracture.